Gypsum-Based Building Products and Method for the Manufacture Thereof

ABSTRACT

A building product comprises calcium sulphate dihydrate particles bound by an organic binder. The calcium sulphate dihydrate particles each have a longest dimension and a lateral dimension, wherein the lateral dimension corresponds to the maximum breadth of the particle about the axis defined by the longest dimension. The calcium sulphate dihydrate particles have a low aspect ratio such that for at least 75% of the calcium sulphate dihydrate particles, the value of the lateral dimension is at least 20% of the value of the longest dimension.

The present invention concerns gypsum-based building products,formulations which can be used to form such building products, and amethod of making such building products from gypsum.

Gypsum is a naturally occurring form of calcium sulphate, in the form ofa stable dihydrate (CaSO₄2H₂O). The term “gypsum”, as used herein, meanscalcium sulphate in that stable dihydrate state; and includes thenaturally occurring mineral, the synthetically derived equivalents (e.g.gypsum obtained from the fuel industry and/or recycling processes), andthe dihydrate material formed by the hydration of stucco (calciumsulphate hemihydrate) or anhydrite.

The properties of gypsum make it highly suitable for use in industrialand building plasters and other building products such as gypsumwallboard. It is a plentiful and generally inexpensive raw materialwhich, through successive steps of dehydration and rehydration, can becast, moulded or otherwise formed to useful shapes. For example, gypsumwallboard; also known as plasterboard or drywall, is formed as a setgypsum core sandwiched between paper cover sheets.

Gypsum is conventionally prepared for use as plaster by grinding andcalcining at temperatures such as from about 120 to 170° C., generallyat atmospheric pressure. This results in partially dehydrated gypsum,typically in the form of the beta crystalline form of the hemihydrate,which may be used as a building or construction material by mixing itwith water to form an aqueous stucco slurry, paste or dispersion, andthen allowing the slurry to set by recrystallisation from the aqueousmedium.

A characterising feature of calcium sulphate particles that are formedthrough this known process of calcination and re-hydration is that theyare needle-shaped, that is, acicular. Effectively, therefore suchparticles have a high aspect ratio, the aspect ratio being the ratio ofthe longest dimension of the particle to its transverse dimension.

Gypsum wallboard generally consists of a hardened gypsum-containing coresurfaced with paper or other fibrous material suitable for receiving acoating such as paint. It is common to manufacture gypsum wallboard bysandwiching an aqueous core slurry predominantly comprising calcinedgypsum between two cover sheets and allowing the aqueous gypsum coreslurry to set or harden by rehydration of the calcined gypsum, usuallyfollowed by heat treatment in a dryer to remove excess water. After thegypsum slurry has set (i.e., reacted with water present in the aqueousslurry) and dried, the formed sheet is cut into required sizes.

However, calcining of gypsum to the partially dehydrated form, and thedrying step, are both energy intensive process and it is therefore agoal to provide a method in which gypsum can be used to make buildingproducts with less embodied energy.

At its most general, the present invention may provide a buildingproduct (e.g. a board for providing a partition) that is formed bymixing a particulate matrix directly with a binder. The bindereffectively glues the matrix together, but it is not expected that anychemical reaction will occur between the binder and the matrix. Thematrix may comprise e.g. calcium sulphate dihydrate, sand, calciumcarbonate, or other inert hard particles. Alternatively, the matrix maycomprise rubber. Typically, the binder is organic. Typically, the binderis a viscoelastic glue.

In general, the matrix provides at least 50 wt %, preferably at least 60wt %, most preferably at least 65 wt % of the building product.

Typically, the matrix is calcium sulphate dihydrate. In this case, themethod allows the energy-intensive steps of calcining and drying to beavoided.

Since the calcium sulphate particles present in the building producthave not undergone the process of calcination and re-hydration, theygenerally do not grow in a needle-shaped form.

Thus, it is normally possible to distinguish building products formedthrough direct mixing of calcium sulphate dihydrate with a binder fromproducts formed through the known process of calcination andre-hydration, by considering the shape of the calcium sulphate dihydrateparticles, in particular their aspect ratio. Typically, calcium sulphatedihydrate particles in a product formed by direct mixing of theparticles with a binder have a more block-like shape than theneedle-shaped particles formed in the known process of calcination andre-hydration.

Therefore, in a first aspect, the present invention may provide abuilding product comprising calcium sulphate dihydrate particles boundby an organic binder,

-   -   the calcium sulphate dihydrate particles each having a longest        dimension and a lateral dimension, the lateral dimension        corresponding to the maximum breadth of the particle about the        axis defined by the longest dimension,    -   the calcium sulphate dihydrate particles having a low aspect        ratio such that for at least 75% of the calcium sulphate        dihydrate particles, the value of the lateral dimension is at        least 20% of the value of the longest dimension.

The calcium sulphate particles provided in a product according to thisaspect of the invention may be e.g. plate-like or block-like, ratherthan needle-like. Effectively, therefore, the calcium sulphate particleshave a low aspect ratio, for example, an aspect ratio lower than 10, incertain cases lower than 5, in other cases lower than 3.

The lateral dimension extends crossways to the longest dimension, forexample, perpendicularly to the longest dimension.

Generally, at least 85% of the calcium sulphate dihydrate particles havea lateral dimension that is at least 20% of the longest dimension.

Typically, at least 75%, in some cases at least 85%, of the calciumsulphate dihydrate particles have lateral dimension that is at least 30%of the longest dimension. In certain cases, at least 75% of the calciumsulphate dihydrate particles have lateral dimension that is at least40%, in some cases at least 50% or at least 60%, of the longestdimension.

Typically, the D₅₀ particle size of the calcium sulphate dihydrateparticles (that is, the equivalent particle diameter value that isgreater than the equivalent particle diameter of exactly 50 wt % of theparticles) is greater than 3 μm, in certain cases greater than 20 μm orgreater than 50 μm. In general, the D₅₀ particle size is less than 2 mm,in certain cases less than 0.5 mm, in some cases less than 250 μm. TheD₅₀ size of the calcium sulphate dihydrate particles may be measurede.g. by laser granulometry.

Typically, the building product is a self-supporting body. For example,it may be a monolithic element such as a gypsum board. The term“self-supporting body” excludes coatings (such as those produced throughthe application of jointing compounds) since coatings are notself-supporting, but are instead supported on a substrate. In general,the building product is a rigid body.

In certain embodiments, the building product may have an approximatelyplanar shape (that is, its size along one dimension is considerablylower than along the remaining two dimensions). In alternativeembodiments, the building product may have a blocky shape. Typically,the volume of the building product is greater than 0.005 m³, preferablygreater than 0.007 m³, more preferably greater than 0.01 m³.

The gypsum particles used in this building product are preferablynatural. Alternatively, they may be derived from an industrial source,provided that they are in the form of the dihydrate. In certain cases,the gypsum may be a mixture of natural and synthetic calcium sulphatedihydrate. For example, the calcium sulphate dihydrate particles maycomprise gypsum formed through flue gas desulphurization.

Typically, the binder may include a vegetable-derived organic binder,such as an alkyd resin, for example, one based on triglycerides derivedfrom polyunsaturated fatty acids from plant or vegetable oils (such as,linseed oil). Other suitable such organic binders include soy protein,pine resins (such as rosin, modified rosins and their derivatives,poly-terpene, modified terpenes, terpene-phenol resins or rosin esterresins), starch and starch derivatives, and modified natural rubberlatex.

Starch binders may include pre-gelatinised starches and nonpre-gelatinised starches. A preferred example of a pre-gelatinisedstarch is Staramic 747™. A preferred example of a non pre-gelatinisedstarch is native corn starch.

Preferably, the binder comprises starch or soy protein. The organicbinder may be a water-based glue.

In certain cases, the binder may comprise an adhesive composition ofpolyaminopolyamide-epichlorohydrin (PAE) resin and protein, such as thatdescribed in US 2008/0050602.

In certain cases, the binder may comprise a mixture of organic bindercompounds. The binder may comprise natural and/or synthetic compounds.

In certain cases, the binders may be provided in a modified,cross-linked form. Cross-linking may be induced through addition,elimination, substitution, condensation, mineral or metal complexation,radiation curing, and/or a coupling reaction, as is known in the art.

Although the binder is typically a vegetable-derived organic binder,other forms of binder are also possible. For example, the binder may bea synthetic organic binder, e.g. it may be derived from petrochemicals.For example, the binder may be an adhesive composition such as describedin CA1230695, which is hereby incorporated by reference.

In this case, the binder may comprise, for example, aliphatic oraromatic hydrocarbon resins, phenol resins, xylene resins, orcoumarone-indene resin. Such resins may be provided in the form of anadhesive composition, further comprising one or more of the followingcomponents: an aqueous dispersion of a natural or synthetic polymer, aninorganic filler, a fibrous filler, and a surfactant.

Typically, the binder does not comprise cellulose or cellulosicthickeners, such as may be found in jointing compounds. In other cases,cellulose or cellulosic thickener is present in an amount of less than0.2 wt % of the building product, preferably less than 0.1 wt %.

Typically, the building product comprises at least 0.5 wt % organicbinder, preferably at least 1 wt %, in certain cases at least 5 wt %. Ingeneral, the building product comprises less than 30 wt % organicbinder, preferably less than 20 wt %, more preferably less than 15 wt %.

In general, the building product comprises at least 50 wt % calciumsulphate dihydrate particles, preferably at least 60 wt %, morepreferably at least 65 wt %. Typically, the building product comprisesless than 99 wt % calcium sulphate dihydrate particles, preferably lessthan 95 wt %, more preferably less than 90 wt %.

The building product according to the first aspect of the invention mayfurther comprise an inorganic binder e.g. cement, lime stucco, oranother pozzolanic binder. The inorganic binder is typically present inan amount up to 20 wt % of the building product, preferably up to 15 wt%.

The presence of inorganic binder is not essential, as the strength ofthe building product derives mainly from the organic binder. In fact, incertain embodiments of the invention, there is no inorganic binderpresent, or less than 1 wt % inorganic binder.

In certain cases, the binder may comprise a mixture of two or morematerials, e.g. an organic material mixed with an inorganic material.

Because gypsum has a density of 2.32 kg/m³, it is frequently desirableto reduce the density of gypsum-based product by the addition oflightweight fillers, of which those listed in the following Table aregiven by way of example:

TABLE Filler Density (kg/m³) Expanded 0.02 polystyrene (EPS) Cenisphere0.4-0.8 Perlite 0.03-0.15 Vermiculite 0.45-1.05 Other expanded clay0.33-0.48 Cork 0.11-0.21 Rubber 0.5  Wood (chip) 0.7  Bagasse 0.12 Bran0.25 Saw dust 0.21 Natural fibres 1.5  Aerocellulose 0.06-0.3 Ecocradle* 0.11-0.33 Greensulate* 0.17-0.3  Glass bubble 0.1-0.3*Ecocradle and Greensulate are trade marks

The lightweight filler is typically provided in particulate form, theparticle size generally lying in the range 30 μm to 4 mm.

Other filler materials, such as silica sand, dolomite or calciumcarbonate may also be added.

As a result of its composition, the building product typically has adensity that is broadly comparable to that of conventional plasterboard.That is, lowest density of the building products according to the firstaspect of the invention may be in the region of 250 kg/m³, possibly inthe region of 300 kg/m³ or 400 kg/m³. In these cases, the process offormation of this building product may typically include the addition ofstabilised foam to the calcium sulphate dihydrate and binder mixture.

Typically, the highest density of building products according to thefirst aspect of the invention is in the region of 1600 kg/m³, possiblyin the region of 1400 kg/m³ or 1300 kg/m³.

Densities lying in the range between the highest and lowest values citedare also possible.

The building product may be in the form of a board. In this case, theboard may further include reinforcing fibres. Such reinforcing fibresmay include glass fibres (typically cut fibres) or other fibresconventionally employed in gypsum board. Such a board according to theinvention may be with or without surface reinforcement or liner sheets;when surface reinforcement is used, it may, for example, be of fibrescrim, fibre mesh, paper, glass mat, polyester mat, natural fibre mat(e.g. hemp, wood, bamboo, or flax), cotton/polyester mat,polyester/glass mat. In certain embodiments, a liner sheet may have ahoneycomb structure. Preferably, the liner sheet comprises 3-ply paper.

Other non-deleterious materials, adjuvants and ingredients may, whenappropriate, be present in the building product. Such non-deleteriousmaterials may include optional further ingredients, such as moisturerepellents (such as silicone oils or waxes, bactericides, fungicides,colouring agents and fire retardants), fluidisers, surfactants (e.g.anionic surfactants, cationic surfactants, or a mixture of both),stabilised foam, adsorbents for volatile organic compounds (that is, VOCscavengers), and siccative agent. The presence of siccative agent may beparticularly preferred when the binder comprises alkyd resin.

In a second aspect, the present invention provides a method ofmanufacturing a gypsum-based product, comprising the steps of:

-   -   providing a quantity of particles of calcium sulphate dihydrate;    -   providing an organic binder for binding the particles of calcium        sulphate dihydrate together; and    -   mixing the particles of calcium sulphate dihydrate with the        binder.

In general, the method comprises the further step of allowing themixture of calcium sulphate dihydrate and binder to cure to provide abuilding product that is a self-supporting body.

The binder may be provided in powder form, or as an aqueous solution oran aqueous emulsion/dispersion. In general, the method includes the stepof curing the binder, to ensure bonding between gypsum particles.

The method of the second aspect of the invention may comprise thefurther step of adding an inorganic binder, e.g. cement, lime stucco, oranother pozzolanic binder. It is thought that use of an inorganic bindermay enhance mechanical properties of the gypsum-based product, inparticular, the early strength and setting of the product.

The gypsum used according to the invention is preferably natural and maybe crushed or ground before mixing with the binder.

Because gypsum has a density of 2.32 kg/m³, it is frequently desirableto reduce the density of gypsum-based product by the addition oflightweight fillers, such as those listed in relation to the firstaspect of the invention.

In addition to this, or as an alternative, the density of thegypsum-based product may be lowered through the addition of foam.

The components used in the formation of a building product according tothe method of the second aspect of the invention may each individuallycomprise optional features of the components present in the buildingproduct of the first aspect of the invention.

The building product formed through the method of the second aspect ofthe invention may have any of the optional features of the buildingproduct of the first aspect of the invention.

In a third aspect, the present invention may provide a gypsumformulation comprising

i) finely divided gypsum (that is, calcium sulphate dihydrate) as apredominant proportion by weight of the formulation; andii) as binder for the gypsum, a vegetable-derived organic adhesivematerial, in a minor proportion by weight of the formulation.

The components of the gypsum formulation according to the third aspectof the invention may each individually comprise optional features of thecomponents present in the building product of the first aspect of theinvention.

Certain advantageous features of the invention and the way it can be putinto operation are now illustrated in the following worked illustrativeExamples, with reference to the following Figures:

FIG. 1 is a scanning electron micrograph of a building product accordingto a first embodiment of the first aspect of the invention.

FIG. 2 is a scanning electron micrograph of a building product accordingto a second embodiment of the first aspect of the invention.

FIG. 3 is a scanning electron micrograph of a comparative example of agypsum-based building product.

FIG. 4 shows the building product of FIG. 3 at a higher magnification.

FIG. 5 shows the building product of FIG. 3 at a lower magnification.

FIG. 6 shows a typical particle size distribution for gypsum particlesin a composite specimen according to an embodiment of the presentinvention.

EXAMPLE 1

Test specimens in the shape of prisms were prepared from gypsum, astarch binder, and optionally, a filler material. The starch was eithera non pre-gelatinised starch (in this case, a native corn starch), orone of two pre-gelatinised starches (pre-gelatinised starch 1=staramic747 from Tate & Lyle; pre-gelatinised starch 2=ICB 1300 from Tate &Lyle). The density and flexural strength were measured and are set outin Table 1.

TABLE 1 flexural strength composition density (kg/m³) (N/mm²) 100 partsof gypsum- 30 μm 646 1.1 5 parts of pregelatinised starch 1 1.5 part ofEPS 100 parts of gypsum- 30 μm 1395 1.32 0.13 parts of pregelatinisedstarch 1 100 parts of gypsum- 30 μm 1488 5.6 2 parts of pregelatinisedstarch 1 100 parts of gypsum- 30 μm 1430 9.8 5 parts of pregelatinisedstarch 1 100 parts of gypsum- 100 μm 1330 5.6 5 parts of pregelatinisedstarch 1 100 parts of gypsum- 1 mm 1243 1.3 5 parts of pregelatinisedstarch 1 100 parts of gypsum- 30 μm 1397 5.4 2.5 parts of nonpregelatinised starch 100 parts of gypsum- 30 μm 1372 9.5 5 parts of nonpregelatinised starch 100 parts of gypsum- 30 μm 798 3.1 5 parts of nonpregelatinised starch 35 parts of Expended Clay (2-4 mm) 100 parts ofgypsum- 30 μm 1082 5.1 5 parts of non pregelatinised starch parts ofrubber 100 parts of gypsum- 30 μm 1085 4.5 5 parts of non pregelatinisedstarch 12.5 parts of Expended clay 100 parts of gypsum- 30 μm 1103 3.5 5parts of non pregelatinised starch 25 parts of Expended clay 100 partsof gypsum- 30 μm 769 1.2 5 parts of pregelatinised starch 2 Lightenedwith foam 100 parts of gypsum- 30 μm 734 1.2 5 parts of pregelatinisedstarch 2 Lightened with foam 100 parts of gypsum- 30 μm 652 0.8 5 partsof pregelatinised starch 2 Lightened with foam 100 parts of gypsum- 30μm 592 0.5 5 parts of pregelatinised starch 2 Lightened with foam

EXAMPLE 2

Test specimens in the shape of prisms were prepared from gypsum, analkyd resin binder, a filler material, and optionally, a siccativeagent. The alkyd resin is a commercial product from Cray Valley. Asiccative agent from OMG Borcher was used for several examples. Thedensity and flexural strength were measured and are set out in Table 2.

TABLE 2 flexural strength composition density (kg/m3) (N/mm²) 100 partsof gypsum- 30 μm 1013 5.15 5 parts of Synaqua 4804 0.3 parts ofsiccative 0.5 part of EPS 100 parts of gypsum- 30 μm 825 3.81 5 parts ofSynaqua 4804 0.3 parts of siccative 1. parts of EPS 100 parts of gypsum-30 μm 645 2.3 5 parts of Synaqua 4804 0.3 parts of siccative 1.5 partsof EPS 100 parts of gypsum- 30 μm 578 1.04 5 parts of Synaqua 4804 0.3parts of siccative 2 parts of EPS 100 parts of gypsum- 30 μm 988 4.99 5parts of Synaqua 4804 0.5 parts of EPS 100 parts of gypsum- 30 μm 8253.81 5 parts of Synaqua 4804 1 parts of EPS 100 parts of gypsum- 30 μm629 2.21 5 parts of Synaqua 4804 1.5 parts of EPS 100 parts of gypsum-30 μm 563 2.2 5 parts of Synaqua 4804 2 parts of EPS 100 parts ofgypsum- 30 μm 1087 3.16 3 parts of Synaqua 4804 0.5 part of EPS 100parts of gypsum- 30 μm 876 2.24 3 parts of Synaqua 4804 1 part of EPS100 parts of gypsum- 30 μm 769 2.34 3 parts of Synaqua 4804 1.5 parts ofEPS 100 parts of gypsum- 30 μm 680 1.76 2 parts of EPS 100 parts ofgypsum- 30 μm 562 1.50 3 parts of Synaqua 4804 2.5 parts of EPS 100parts of gypsum- 30 μm 758 1.13 5 parts of Synaqua 4804 Lightened withfoam 100 parts of gypsum- 30 μm 640 0.87 5 parts of Synaqua 4804Lightened with foam 100 parts of gypsum- 30 μm 580 0.66 5 parts ofSynaqua 4804 Lightened with foam

EXAMPLE 3

Test specimens in the shape of prisms were prepared from gypsum, aprotein-based binder (Soyad, developed by the Hercules group), and afiller material. The density and flexural strength were measured and areset out in Table 3.

TABLE 3 flexural strength composition density (kg/m3) (N/mm²) 100 partsof gypsum- 30 μm 1066 1.19 3 parts of Soyad 0.5 part of EPS 100 parts ofgypsum- 30 μm 869 0.8 3 parts of Soyad 1. parts of EPS 100 parts ofgypsum- 30 μm 733 0.61 3 parts of Soyad 1.5 parts of EPS 100 parts ofgypsum- 30 μm 642 0.51 3 parts of Soyad 2 parts of EPS 100 parts ofgypsum- 30 μm 962 3.48 10 parts of Soyad 0.5 parts of EPS 100 parts ofgypsum- 30 μm 748 2.31 10 parts of Soyad 1 parts of EPS 100 parts ofgypsum- 30 μm 645 1.74 10 parts of Soyad 1.5 parts of EPS 100 parts ofgypsum- 30 μm 572 1.5 10 parts of Soyad 2 parts of EPS 100 parts ofgypsum- 30 μm 875 4.1 13 parts of Soyad 0.5 part of EPS 100 parts ofgypsum- 30 μm 742 3.58 13 parts of Soyad 1 part of EPS 100 parts ofgypsum- 30 μm 643 2.66 13 parts of Soyad 1.5 parts of EPS 100 parts ofgypsum- 30 μm 643 2.36 13 parts of Soyad 2 parts of EPS 100 parts ofgypsum- 30 μm 759 2.25 13 parts of soyad Lightened with foam 100 partsof gypsum- 30 μm 838 4.17 13 parts of soyad Lightened with foam 100parts of gypsum- 30 μm 796 3.19 13 parts of soyad Lightened with foam100 parts of gypsum- 30 μm 636 2.05 13 parts of soyad Lightened withfoam 100 parts of gypsum- 30 μm 694 2.08 13 parts of soyad Lightenedwith foam 100 parts of gypsum- 30 μm 744 2.94 13 parts of soyadLightened with foam

EXAMPLE 4

Test specimens in the shape of prisms were prepared from gypsum, apine-derived binder (terpene phenol emulsion TR602), and a fillermaterial. The density and flexural strength were measured and are setout in Table 4.

TABLE 4 flexural strength composition density (kg/m3) (N/mm²) 100 partsof gypsum- 30 μm 1075 1.25 3 parts of TR602 0.5 part of EPS 100 parts ofgypsum- 30 μm 842 1.09 3 parts of TR602 1. parts of EPS 100 parts ofgypsum- 30 μm 726 1.28 3 parts of TR602 1.5 parts of EPS 100 parts ofgypsum- 30 μm 631 078 3 parts of TR602 2 parts of EPS 100 parts ofgypsum- 30 μm 1093 2.05 5 parts of TR602 0.5 parts of EPS 100 parts ofgypsum- 30 μm 903 1.92 5 parts of TR602 1 parts of EPS 100 parts ofgypsum- 30 μm 747 1.87 5 parts of TR602 1.5 parts of EPS 100 parts ofgypsum- 30 μm 656 1.51 5 parts of TR602 2 parts of EPS

Measurement of Flexural Strength

After production, the prisms were dried for 24 hours at 40° C. andsubsequently conditioned at 25° C. and 50% humidity for a further 24hours. The flexural strength of the prisms was measured underthree-point bending using a Zwick universal testing machine.

Production of Prisms

The following protocol was used to produce prisms containing a starchbinder. An analogous method was used for prisms containing otherbinders.

-   -   1. Blend the powders: gypsum and additives (light weight fillers        for example)    -   2. Mix water with powder starch    -   3. Pour the gypsum powder blend into the water/starch mixture    -   4. Mix the powder blend and the water in a mixer (such as        Kenwood mixer) until having a homogeneous slurry    -   5. If the sample comprises foam as a light weight agent, the        foam, generated separately according to processes known in the        art, is preferably added at this stage    -   6. Pour the slurry into the moulds    -   7. Skim the excess of gypsum slurry    -   8. Put the mould on a tray. The mould might be covered by a        metal tray.    -   9. Cure/dry the samples in the mould for a suitable time and at        a suitable temperature, e.g. 1 h30 at 140° C., 24 h at 40° C.,        or a time between 1 h30 and a temperature between 40° C. and        140° C.

The mechanical performance of the samples was tested at roomtemperature. The samples were held room temperature (15-25° C.) for atleast a day before testing.

This method may be used for making boards having a thickness of ˜13 mm.In this case the curing step is: 1 h10 at 120° C. Moreover, it ispossible to add a paper liner on both sides before and after pouring thegypsum paste as is known in the art.

Scanning Electron Micrographs

FIGS. 3, 4, and 5 show scanning electron micrographs of gypsumplasterboards produced according to a known calcination and re-hydrationprocess. It can be seen that this process results in the formation ofneedle-like gypsum crystals, having a high aspect ratio. The length ofthe needles is approximately 20 μm.

By contrast, FIGS. 1 and 2 show scanning electron micrographs of gypsumcomposite according different embodiments of the present invention. FIG.1 shows a composite having a soy protein binder, while FIG. 2 shows aplasterboard having a starch binder. The gypsum crystals correspond tothe pale areas of the micrograph, while the dark areas correspond to thebinder and/or the epoxy resin used to mount the samples.

It can be seen that in gypsum plasterboards according to the presentinvention, the gypsum particles are more block-like than needle-like.Furthermore, the size of the gypsum particles in building products shownin FIGS. 1 and 2 is much greater than that of the gypsum needles presentin the comparative example shown in FIGS. 3 to 5.

FIG. 6 shows a typical particle size distribution for gypsum particlesin a composite specimen according to an embodiment of the presentinvention. From this it can be seen that in general gypsum particles inthe composite specimen have a particle size between 1 and 100 μm. Themajority of particles (measured by volume) have a particle size between20 and 60 μm.

1. A building product comprising calcium sulphate dihydrate particlesbound by an organic binder, the calcium sulphate dihydrate particleseach having a longest dimension and a lateral dimension, the lateraldimension corresponding to the maximum breadth of the particle about theaxis defined by the longest dimension, the calcium sulphate dihydrateparticles having a low aspect ratio such that for at least 75% of thecalcium sulphate dihydrate particles, the value of the lateral dimensionis at least 20% of the value of the longest dimension.
 2. A buildingproduct according to claim 1, wherein the value of the lateral dimensionis at least 40% of the value of the longest dimension.
 3. A buildingproduct comprising calcium sulphate dihydrate particles bound by anorganic binder, wherein 90 wt % of the calcium sulphate dihydrateparticles have a particle size in the range of 1 μm to 3 mm.
 4. Abuilding product according to claim 1, wherein the building product is aself-supporting body.
 5. A building product according to claim 1,wherein the binder is a vegetable-derived organic binder that isoptionally modified through cross-linking.
 6. A building productaccording to claim 5, wherein the binder comprises one or more of thefollowing: soy and/or a derivative of soy; starch and/or a starchderivative.
 7. A building product according to claim 1, wherein thebinder is present in an amount of 0.5-30 wt % of the product.
 8. Abuilding product according to claim 1, wherein the calcium sulphatedihydrate particles are present in an amount of 50-90 wt % of theproduct.
 9. A building product according to claim 1, having a density inthe range 250-1600 kg/m³.
 10. A building product according to claim 1,further comprising an inorganic binder, the inorganic binder optionallycomprising a cementitious material.
 11. A building product according toclaim 10, wherein the inorganic binder is present in an amount of up to20 wt %.
 12. A method of manufacturing a gypsum-based product,comprising the steps of: providing a quantity of particles of calciumsulphate dihydrate; providing a binder for binding the particles ofcalcium sulphate dihydrate together; and mixing the calcium sulphatedihydrate and the binder.
 13. A method according to claim 12, comprisingthe further step of allowing the mixture of calcium sulphate dihydrateand binder to cure to provide a building product that is aself-supporting body.
 14. A method of manufacture according to claim 12,wherein the binder is provided in powder form, and the binder optionallycomprises water.
 15. A method of manufacture according to claim 12,further comprising the step of providing a filler material for mixingwith the calcium sulphate dihydrate and the binder.
 16. A method ofmanufacture according to claim 12, further comprising the step of addinga stabilized foam to the mixture of calcium sulphate dihydrate andbinder.
 17. A building product prepared according to the method ofmanufacture of claim 12.